A frame transfer CCD image sensor in general includes three sections: a photosensing array, known as the A-register; a temporary storage array, known as the B-register; and an output register, known as the C-register. The three registers are formed along a major surface of a substrate of semiconductor material. It is the practice to mask out the B-register and the C-register so that light from the image being sensed will only impinge the A-register. In one type of CCD image sensor the light enters the semiconductor substrate through its surface opposite to the surface along which the registers are formed, which surface is generally referred to as the back surface of the substrate. In order to allow the light to pass readily through the substrate to the A-register, the substrate is thinned from its back surface and a glass plate is mounted on the back surface to provide support for the thinned substrate. This type of device is described in U.S. Pat. No. 4,266,334 to T. W. Edwards et al., issued May 12, 1981, entitled "Manufacture of Thinned Substrate Imagers".
In this type of imager the mask used for the B-register and C-register has been a metal film coated on the surface of the glass plate. The metal film has been applied to the glass plate using a metal masking plate which is placed over the glass plate, the openings in the masking plate are aligned with the areas of the CCD imager to be covered. The metal is then applied to the glass plate through the openings in the mask plate. This technique of applying the masking film has a number of problems. One problem is achieving good alignment between the masking plate and the CCD image sensor so that the metal film will properly cover the desired areas of the CCD image sensor. Another problem is that the metal masking film is spaced from the CCD image sensor substrate by the glass plate which allows some light to seep under the edge of the masking film through the glass plate. The second problem could be solved by placing the metal masking film directly on the back surface of the CCD image sensor substrate. However, because the CCD image sensor substrate is so thin, about 10 micrometers in thickness, it is very difficult to handle the substrate to apply the metal film directly thereto without breaking the substrate.